The present invention relates to time-of-flight mass spectrometers.
In a time-of-flight mass spectrometer charged particles are analysed depending on their mass-to-charge ratios. This is accomplished by measuring the time difference between ions leaving an ion source and arriving at an ion detector. In known time-of-flight mass spectrometer arrangements ions created in an ion source are introduced into a field-free drift space and are reflected using an ion reflector. An ion reflector has a series of parallel electrodes which generates an electric field for reflecting ions back into the field-free drift space to be detected by the ion detector. Ions are pulsed or bunched in time at certain points downstream from the ion source to have smaller time deviations compared to their flight times. However, the ions will usually have a range of different kinetic energies, and so velocities, resulting in an undesirable spread of flight times, The ion reflector is used to compensate for this spread of flight times. The ions with larger velocities penetrate further into the ion reflector spending more time there before being reflected into the field free drift space where they spend less time. The electric field strength is chosen so that the increased and decreased times match to cancel each other.
An ion reflector having a uniform or linear electric field is called a single-stage reflector. This compensates for a spread of flight times, up to the first derivative of ion energy and so will only provide effective compensation for a relatively small range of energy spread. Single-stage reflectors have been successfully used in a wide range of applications, but are limited in their effectiveness.
Another type of the ion reflector which provides a wider range of energy compensation uses two stages each having a uniform electric field separated by a fine grid mesh. This is called a dual-stage reflector. In a dual-stage reflector, a short first stage reduces the initial energy of the ions by more than two thirds and has very high electric field strength. The ions, with now only less than one third of their initial energy, are reflected in the low electric field second stage, and this provides effective compensation for a spread of flight times up to the second derivative of ion energy.
The dual-stage reflector was first developed by Mamyrin et al. (B. A. Mamyrin, V. I. Karataev, D. V. Shmikk and V. A. Zagulin, Zh. Eksp. Teor. Fiz. 64, (1973) 82-89; Sov. Phys. JETP., 37 (1973) 45-48). It was believed that the best resolution could be obtained if the first stage is very short and has quite a high field strength compared to the second stage, i.e. the ratio of the electric field in the low-field second stage to the electric field in the high-field first stage is small. Typically, the first stage had a length of about 10% of the total reflector length. This is borne out by theory because the resolution derived from the condition for second order compensation is proportional to the ratio of the ion energy at the boundary of the two stages to the initial ion energy at the front of the reflector. The theoretical maximum for this ratio is one third, in which case the first stage length is infinitely small and the field strength of the first stage is infinitely large. Thus, the first stage length is selected so as to be as short as possible unless practical limitations, such as electric discharge or mesh size effect, cause serious problems. In practice, the energy reduction at the boundary of the two stages was set to be less than about 0.7 of the initial ion energy which is slightly larger than two thirds, and the ratio of the field strengths in the two stages was below 0.25.
The dual-stage reflector has excellent mass resolution, and is quite useful in most of the high resolution applications currently encountered. However, the requirement for a mesh or grid to separate the two stages of uniform electric field and also separating the reflector from the field-free drift space reduces the sensitivity of the apparatus because the ions must pass through a mesh or grid four times, thereby suffering substantial scattering and deflection. U.S. Pat. No. 4,731,532 describes an ion reflector designed to alleviate the reduced sensitivity by removing the grids or meshes, as shown in FIG. 1. However, the high field strength in the first stage causes field penetration into the second stage and also into the field-free drift space causing the equi-potential lines bent at both ends of the first stage. This bending of the field lines deflects the ions resulting in a shift of their flight times. These effects are corrected by attaching an additional electrode, called the focusing electrode, to the front of the first stage in order to alleviate the undesirable ion dispersion.
Other types of gridless reflector have been used for different purposes where there is a need to correct for a much wider range of energy spread. U.S. Pat. No. 4,625,112 describes an ion reflector which uses a quadratic electric field to reflect the ions and which, in theory, provides for perfect temporal correction provided there is no field-free drift space. U.S. Pat. No. 5,464,985 discloses an ion reflector using a curved electric field. Both of these patents embody an increasing electric field starting from zero, or close to zero, at the front end of the reflector so that the field distortion caused by using gridless electrodes will be small compared to that produced in a gridless dual-stage reflector. On the other hand, an electric field strength which increases along the reflector axis gives rise to small but successive ion divergence, and this reduces the sensitivity.
It is an object of the present invention to provide a gridless dual-stage ion reflector which substantially alleviates the aforementioned problems.
It is another object of the invention to provide a convenient method for adjusting the time focal plane of a gridless dual-stage ion reflector whereby to correct for any misalignment.
According to one aspect of the invention there is provided a time-of-flight mass spectrometer comprising an ion source for generating an ion beam, a field-free drift region, a gridless dual-stage ion reflector and an ion detector for generating a signal indicative of the ion beam, the gridless dual-stage ion reflector including a plurality of disc electrodes having central apertures through which the ion beam can pass and a final plate electrode, said electrodes being supplied, in use, with voltages defining a high-field first stage having a substantially uniform electric field, and a low-field second stage also having a substantially uniform electric field, the field strength of the second stage having a ratio to that of the first stage in the range from 0.35 to 0.7.
According to another aspect of the invention there is provided a gridless dual-stage ion reflector comprising a plurality of disc electrodes having central apertures through which an ion beam can pass and a final plate electrode, the electrodes being supplied, in use, with voltages defining a high-field first stage having a substantially uniform electric field and a low-field second stage also having a substantially uniform electric field, the field strength of the second stage having a ratio to that of the first stage in the range from 0.35 to 0.7.
According to a yet further aspect of the invention there is provided a method for adjusting the time focal plane of a gridless, dual-stage ion reflector as defined in the immediately preceding paragraph, including adjusting in opposite directions said voltages defining said first and second stages to set a selected ratio of the field strength of the second stage to that of the first stage in said range from 0.35 to 0.7, and then adjusting the voltages in the same direction, while maintaining said selected ratio.
As a result of investigations carried out by the inventor, it was found that the hitherto-used grid or mesh electrodes provided at the front of a dual-stage ion reflector and at the boundary between the first and second stages can be replaced by gridless electrodes, and that the difference in field strengths in the two stages can be reduced significantly if the first stage is not too short. In fact, it was found that the ratio of electric field strength in the low-field second stage to the electric field strength in the high-field first stage is optimally 0.55, and that ratios in the range 0.35 to 0.7 are also useful. By adopting a ratio in this range, distortion of the equipotential lines due to field penetration from the higher field stage is reduced.
Inevitably there will be some penetration of the higher field strength of the first stage into the field-free drift space and into the second stage which has a lower field strength and this gives rise to a small shift in the time focal plane. However, this small shift can easily be compensated for by applying a small change to the ratio of the field strengths in the two stages. Another important finding is that the longer first stage, compared to a known dual-stage reflector, provides relatively small field penetration or, in other words, lower distortion of the equi-potential lines at both ends of the first stage. These distortions can be regarded an aperture lenses which can be used to change the trajectories of ions to be focused into the ion detector.
A parallel ion beam is caused to diverge on entering the first high-field first stage. This beam continues to diverge due to deceleration in the first stage. However, at the boundary of the first and second stages the ion beam is caused to converge and can be made parallel to the reflector axis by carefully choosing the ratios of the first stage length and of the diameter of the central apertures with respect to the length of the field-free drift space, and the ratio of the field strengths in two stages. The diameter of the central apertures may be within a range from 0.02 to 0.04 of the total free flight length and/or the length of the first stage may be within a range from 0.04 to 0.1 of the length of the total free flight length. Then, the ion beam is reflected parallel to the axis again in the second stage and follows the same trajectory on the way back. This situation is useful because the focal length of the reflector becomes infinite and ions originating at the ion source will maintain the same angle to the reflector axis after reflection by the ion reflector and will approach the ion detector without divergence. Additionally, by slightly reducing the first stage length a lens power due to field distortion at the boundaries becomes slightly stronger. This modification causes the reflector to act like a weakly converging lens which is quite useful to spatially focus the ion beam in to the ion detector.
It was also confirmed that the described ion reflector can be used in the configuration where the reflector is inclined from the axis of the ion beam penetrating into the reflector and the ion detector is positioned off-axis accordingly. A small shift in the time focal plane which determines the flight path can again be compensated for by adjusting the field strength. The ion detector may have a detector surface perpendicular to the ion beam axis, or may be inclined at an angle which is twice as large as the angle of inclination of the reflector, or may be at a small angle around that angle. Adjusting the field strength can also compensate for this change.